DCT designers typically are concerned with improving the efficiency of a power amplifier (PA) at its maximum output power level, as this efficiency improvement is important in order to reduce the current/power consumption at high transmission power. However, a mobile station transmitter typically does not operate at its maximum output condition. For example, according to a CDMA CDG4 statistical profile (see CDMA Development Group, “CDG System Performance Tests (Optional)”, Rev. 3.0 draft, Apr. 9, 2003) the most often encountered transmission power for a CDMA mobile station is at the mid to low power level, such as between about +3 and −10 dBm for a CDMA mobile transmitter used for voice communications. In this transmission power region, the transmitter (Tx) chain dominates the power consumption of the overall transmitter. It is apparent that reducing the Tx chain current consumption can efficiently increase talk time of the mobile station by conserving battery power.
In at least some conventional mobile station transmitter designs the level of the base-band (BB) signal coming from a digital to analog converter (DAC) that is applied at the input of the Tx chain is fixed. In addition to this, the BB signal level is quite high, and may exhibit a 2.5 V peak-to-peak voltage swing. This is based on a consideration that as the BB signal level is made higher, the signal-to-noise ratio (SNR) is also higher. However, to handle such a high level input, a quadrature modulator, which forms the properly modulated transmission signal and converts the BB modulation signal to the desired RF transmission signal, and the following variable gain amplifiers (VGAs) and drive amplifier in the Tx chain, are required to consume a significant amount of current in order to maintain sufficient linearity without causing significant distortion. Therefore, the Tx chain typically operates in an inefficient current consumption condition.
In order to reduce the current (power) consumption of the mobile station transmitter, it is known in the art that the PA bias current can be controlled by adjusting a PA reference current or voltage. However, even greater savings in power consumption, and increases in efficiency, are desired.